The evolution of cellular mobile communication networks beyond the third generation and the introduction of new broadband wireless access technologies such as WiMAX open the way to heterogeneous networks with a diversification of RATs (radio access technologies). In many cases providers offering the same RAT compete with each other on the market. Furthermore, a provider may offer services of different RATs to a user. As an example, a provider may provide UMTS (universal mobile telecommunications system) services as well as WLAN (wideband local area network) services. In such a situation the user is interested in having a radio link which offers the best quality of service (QoS) for the application which runs on his terminal device which might be a mobile phone, a notebook or the like.
The best quality of service for a particular application running on the terminal device depends on many factors. Furthermore, the significance of the factors vary in time such that a radio link which is the best one at some time might be outperformed by another radio link later on. As a consequence it is desirable to change the radio link from time to time in order to have always the best quality of service. This change of radio link is commonly called a handover.
In the prior art the desired application determines the RAT. If, for example, a high data rate is desired, e.g. for downloading music or a video file, WLAN is probably the best choice. If however, the user is interested in video telephony, UMTS is a good choice. If the RAT has been specified it is necessary to identify the best radio link with a communication system with this RAT. At a later time a horizontal handover might be necessary when reception conditions change, i.e. a handover without changing the RAT.
To ensure the best quality of service it needs to be known which quality the individual radio links offer. One approach is that the terminal carries out measurements and reports the results to the communication system to which he has already established a connection. The system then determines the best radio link, and arranges a handover to such a radio link if appropriate. This works quite well when the radio links are always of the same type, i.e. when the radio link connects the terminal device with communication systems using the same RAT. In this case the approach has one drawback: the more radio links become available the more signalling between the terminal device and the communication system is necessary. This however worsens the scalability, i.e. the ratio between the signalling load and the working load.
The approach mentioned in the penultimate paragraph may lead to situations where radio links of a first type remain rather unused, and whereby radio links of a second type are heavily asked for. An example would be a situation where only a few users ask for UMTS links, e.g. for video telephony, but many user ask for WLAN links, e.g. for music download. In this case the data rate for the WLAN links may drop to a low value which could also be offered by a UMTS link. In this case a vertical handover from the WLAN link to the UMTS link would be appropriate to guarantee the best quality of service.
A problem with vertical handovers is that each RAT has its own definition which specify the quality of a radio link. When a terminal device is expected to operate in a multi-RAT environment the quality of two radio links can hardly be compared. Comparing the measurement results can, if at all, only be done with complex algorithms which map such values of a first RAT into values which can be understood by a second RAT. Furthermore, numerous algorithms of this kind are necessary to compare measurement values of several RATs with each other. The signalling of the measurement results mentioned in the last paragraph and the processing of the complex algorithms consume a significant time such that in many cases a handover takes a few seconds. As a consequence, seamless handovers in a multi-RAT environment are difficult to achieve.
In the document A. Festtag, “Optimization of handover performance by link layer triggers in IP-based networks: parameters, protocol extensions and APIs for implementation”, TKN technical report TKN-02-014, T U Berlin, version 1.0, August 2002, two phases of a handover process are identified: a handover detection and triggering phase and a handover execution phase. In order to speed up the first phase the authors suggest the definition of a link layer trigger for a handover. A parameter for this link layer trigger is an abstract measure of a signal quality. This abstract measure is obtained by a mapping of RAT-specific measurement values into this abstract measure.